The carrier bodies for automotive exhaust gas catalysts are generally cylindrical in form, with two end faces and a shell, and have a multiplicity of flow passages for the exhaust gases from the internal combustion engines passing through them from the first end face to the second end face, substantially parallel to the cylinder axis. These carrier bodies are also referred to as honeycomb carriers.
The cross-sectional shape of the carrier bodies depends on the installation requirements at the motor vehicle. Carrier bodies with a round, elliptical or triangular cross section are in widespread use. The flow passages are generally square in cross section and are arranged in a dense grid over the entire cross section of the carrier bodies. The passage or cell density of the flow passages varies between 10 and 140 cm−2 depending on the particular application. Honeycomb carriers with cell densities of up to 250 cm−2 and more are in development.
Catalyst carrier bodies obtained by extrusion of ceramic compounds are predominantly used for the purification of automobile exhaust gases. Alternatively, catalyst carrier bodies made from corrugated and wound metal foils are also available. At present, ceramic carrier bodies with cell densities of 62 cm−2 are predominantly used for purifying the exhaust gas from passenger cars. The cross-sectional dimensions of the flow passages in this case are 1.27×1.27 mm2. The wall thicknesses of carrier bodies of this type are between 0.1 and 0.2 mm.
Finely distributed platinum group metals, the catalytic action of which can be modified by compounds of base metals, are generally used to convert the pollutants contained in automobile exhaust gases, such as carbon monoxide, hydrocarbons and nitrogen oxides, into harmless compounds. These catalytically active components have to be deposited on the carrier bodies. However, it is not possible to obtain the extremely fine distribution of the catalytically active components needed by depositing these components on the geometric surfaces of the carrier bodies. This applies equally to nonporous metallic carrier bodies and to porous ceramic carrier bodies. A sufficiently large surface area for the catalytically active components can only be provided by applying a support layer of fine-particle materials with a high surface area to the inner surfaces of the flow passages. This operation is referred to below as coating the carrier body. It is undesirable to coat the shell surface of the carrier bodies, and this should be avoided in order to avoid losses of valuable catalytically active materials.
For coating the carrier bodies with the fine-particle, high surface area materials a suspension of these materials in a liquid phase, generally water, is used. Typical coating suspensions for catalytic applications contain, for example, active aluminium oxides, aluminium silicates, zeolites, silicon dioxide, titanium oxide, zirconium oxide and oxygen-storing components based on cerium oxide, as support materials with a high surface area for the catalytically active components. These materials form the solids fraction of the coating suspension. Furthermore, it is possible to add to the coating suspension soluble precursors of promoters or catalytically active precious metals from the platinum group of the periodic system of the elements. The solids concentration of typical coating suspensions is in the range between 20 and 65% by weight, based on the total weight of the suspension. Their densities are between 1.1 and 1.8 kg/l.
The prior art has disclosed various methods for depositing the support layer on the carrier bodies using the coating suspension. By way of example, the carrier bodies to be coated can be immersed into the coating suspension, or the coating suspension can be poured over them. Furthermore, it is possible to pump or suck the coating suspension into the flow passages of the carrier bodies.
In all cases, excess coating material has to be removed from the carrier bodies, for example by sucking it out or blowing compressed air through the passages in the carrier bodies. This also opens up any passages which have become blocked by coating suspension.
After the coating operation, the carrier body and support layer are dried, and then the support layer is calcined on the carrier body in order to be consolidated and fixed. Then, the catalytically active components are introduced into the coating by impregnation with generally aqueous solutions of precursor compounds of the catalytically active components. Alternatively, it is also possible for the catalytically active components to be added to the coating suspension itself. In this case, there is no need to subsequently impregnate the completed support layer with the catalytically active components.
One important criterion for the coating methods is the coating or loading concentration which they can achieve in one run. This is to be understood as meaning the solids content which remains behind on the carrier body after drying and calcining. The coating concentration is given in grams per litre of volume of the carrier bodies (g/l). In practice, automobile exhaust gas catalysts require coating concentrations of up to 300 g/l. If this quantity cannot be applied in one run with the selected coating method, the coating operation has to be repeated, after drying and if appropriate calcining of the carrier body, until the desired loading has been reached. Often, two or more coating operations with coating suspensions of different compositions are carried out. This results in catalysts which have a plurality of layers with different catalytic functions on top of one another.
DE 40 40 150 C2 describes a method allowing catalyst carrier bodies in honeycomb form to be coated with a support layer or a catalytically active layer uniformly over their entire length. In the following, catalyst carrier bodies in honeycomb form are also referred to as honeycomb carriers. According to the method described in DE 40 40 150 C2, the cylinder axis of the honeycomb carrier is oriented vertically in order to be coated. Then, the coating suspension is pumped into the passages through the lower end face of the honeycomb carrier until it emerges at the upper end face. Next, the coating suspension is pumped out again at the bottom, and excess coating suspension is removed from the carrier body by blowing or sucking, in order to prevent the passages from becoming blocked. This method produces support layers which have a good uniformity over the entire length of the honeycomb bodies.
The coating method described, like any technical process, has a certain fluctuation range for the coating quantity from carrier body to carrier body. The fluctuation range depends on the nature of the coating suspension and on the properties of the honeycomb carriers to be coated.
The fluctuation range of the coating process has a direct influence on the catalytic activity of the finished catalyst, since the catalytic activity is directly dependent, inter alia, on the loading quantity of the catalytically active precious metals. Therefore, to guarantee a minimum activity of the catalysts, it is necessary to ensure that all the catalysts contain at least a target quantity of coating suspension. This means that the carrier bodies, in production, have to be overloaded with coating suspension by half the fluctuation range of the coating process. Therefore, if it is possible to reduce the fluctuation range of the coating process, the degree of overloading required can be lowered accordingly, with consequent savings on expensive precious metals and coating raw materials.